How to implement Infinitus classification in PHP: recursive method and reference method

The content of this article is about the method of realizing Infinitus classification in PHP: the recursive method and the reference method, which have certain reference value. Friends in need can refer to it. I hope it will be helpful to you.

During the interview, I was asked about the design and implementation of Infinitus classification. A common practice is to add a PID field to distinguish the classification to which you belong when creating a table.

The data is in The storage in the database looks like this. How to implement infinite recursion? There are two common methods, recursion and reference algorithm

Recursive algorithm

    /**
     * 递归实现无限极分类
     * @param $array 分类数据
     * @param $pid 父ID
     * @param $level 分类级别
     * @return $list 分好类的数组 直接遍历即可 $level可以用来遍历缩进
     */

    function getTree($array, $pid =0, $level = 0){

        //声明静态数组,避免递归调用时,多次声明导致数组覆盖
        static $list = [];
        foreach ($array as $key => $value){
            //第一次遍历,找到父节点为根节点的节点 也就是pid=0的节点
            if ($value['pid'] == $pid){
                //父节点为根节点的节点,级别为0，也就是第一级
                $value['level'] = $level;
                //把数组放到list中
                $list[] = $value;
                //把这个节点从数组中移除,减少后续递归消耗
                unset($array[$key]);
                //开始递归,查找父ID为该节点ID的节点,级别则为原级别+1
                getTree($array, $value['id'], $level+1);

            }
        }
        return $list;
    }

    /*
     * 获得递归完的数据,遍历生成分类
     */
    $array = getTree($array);

    foreach($array) as $value{
       echo str_repeat(&#39;--&#39;, $value[&#39;level&#39;]), $value[&#39;name&#39;].&#39;<br />&#39;;
    }

//输出结果 无限极分类实现ok
河北省
--邯郸市
----永年区
--武安市
北京市
--朝阳区
----望京
----酒仙桥
--通州区

Quote algorithm

function generateTree($array){
    //第一步 构造数据
    $items = array();
    foreach($array as $value){
        $items[$value['id']] = $value;
    }
    //第二部 遍历数据 生成树状结构
    $tree = array();
    foreach($items as $key => $value){
        if(isset($items[$item['pid']])){
            $items[$item['pid']]['son'][] = &$items[$key];
        }else{
            $tree[] = &$items[$key];
        }
    }
    return $tree;
}

//经过第一步 数据变成了这样
Array
(
    [1] => Array
        (
            [id] => 1
            [pid] => 0
            [name] => 河北省
            [children] => Array
                (
                )

        )

    [2] => Array
        (
            [id] => 2
            [pid] => 0
            [name] => 北京市
            [children] => Array
                (
                )

        )

    [3] => Array
        (
            [id] => 3
            [pid] => 1
            [name] => 邯郸市
            [children] => Array
                (
                )

        )

    [4] => Array
        (
            [id] => 4
            [pid] => 2
            [name] => 朝阳区
            [children] => Array
                (
                )

        )

    [5] => Array
        (
            [id] => 5
            [pid] => 2
            [name] => 通州区
            [children] => Array
                (
                )

        )

    [6] => Array
        (
            [id] => 6
            [pid] => 4
            [name] => 望京
            [children] => Array
                (
                )

        )

    [7] => Array
        (
            [id] => 7
            [pid] => 4
            [name] => 酒仙桥
            [children] => Array
                (
                )

        )

    [8] => Array
        (
            [id] => 8
            [pid] => 3
            [name] => 永年区
            [children] => Array
                (
                )

        )

    [9] => Array
        (
            [id] => 9
            [pid] => 1
            [name] => 武安市
            [children] => Array
                (
                )

        )

)

//第一步很容易就能看懂，就是构造数据，现在咱们仔细说一下第二步
 $tree = array();
 //遍历构造的数据
    foreach($items as $key => $value){
    //如果pid这个节点存在
        if(isset($items[$value['pid']])){
            //把当前的$value放到pid节点的son中 注意 这里传递的是引用 为什么呢？
            $items[$value['pid']]['son'][] = &$items[$key];
        }else{
            $tree[] = &$items[$key];
        }
    }

//这个方法的核心在于引用，php变量默认的传值方式是按指传递
//也就是说 假如说 遍历顺序是 河北省 邯郸市 当遍历到河北省时 会把河北省放到tree中 遍历到邯郸市时 会把邯郸市放到河北省的子节点数组中 但是！！！ 这会儿的tree数组中 河北省已经放进去了 根据php变量按值传递的规则 你并没有更改tree数组中的河北省的数据 所以这里用到了引用传递
//当你对河北省做更改时，tree数组中的河北省也一并做了更改 下面我们做个实验 我们把引用传递去掉，看一下结果

//使用普通传值输出结果
 Array
(
    [0] => Array
        (
            [id] => 1
            [pid] => 0
            [name] => 河北省
        )

    [1] => Array
        (
            [id] => 2
            [pid] => 0
            [name] => 北京市
        )

)
//可以看到 只有河北省和北京市输出出来了 因为他们俩是第一级节点 而且排行1和2，放到$tree数组中之后，没有使用引用传递，那么后续对他俩的子节点的操作都没有在$tree中生效，现在我们更改一下顺序 把邯郸市放到河北省的前面 那么根据咱们的推断 那么邯郸市就应该出现在tree数组里

//邯郸市放到河北省前面的输出结果
Array
(
    [0] => Array
        (
            [id] => 1
            [pid] => 0
            [name] => 河北省
            [son] => Array
                (
                    [0] => Array
                        (
                            [id] => 3
                            [pid] => 1
                            [name] => 邯郸市
                        )

                )

        )

    [1] => Array
        (
            [id] => 2
            [pid] => 0
            [name] => 北京市
        )

)

//果然是这样 那么证明我们的推断是正确的 现在我们把引用传值改回去 再看一下

//使用引用传值输出结果
Array
(
    [1] => Array
        (
            [id] => 1
            [pid] => 0
            [name] => 河北省
            [children] => Array
                (
                    [0] => Array
                        (
                            [id] => 3
                            [pid] => 1
                            [name] => 邯郸市
                            [children] => Array
                                (
                                    [0] => Array
                                        (
                                            [id] => 8
                                            [pid] => 3
                                            [name] => 永年区
                                        )

                                )

                        )

                    [1] => Array
                        (
                            [id] => 9
                            [pid] => 1
                            [name] => 武安市
                        )

                )

        )

    [2] => Array
        (
            [id] => 2
            [pid] => 0
            [name] => 北京市
            [children] => Array
                (
                    [0] => Array
                        (
                            [id] => 4
                            [pid] => 2
                            [name] => 朝阳区
                            [children] => Array
                                (
                                    [0] => Array
                                        (
                                            [id] => 6
                                            [pid] => 4
                                            [name] => 望京
                                        )

                                    [1] => Array
                                        (
                                            [id] => 7
                                            [pid] => 4
                                            [name] => 酒仙桥
                                        )

                                )

                        )

                    [1] => Array
                        (
                            [id] => 5
                            [pid] => 2
                            [name] => 通州区
                        )

                )

        )

)
//树状结构完美的输出出来了 这个方法的核心就是引用传值

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